A communication system including a roadside device placed along a road or the like and a vehicle-mounted device mounted on a vehicle in which road-to-vehicle communication is performed by the roadside device and the vehicle-mounted device and vehicle-to-vehicle communication is performed by the vehicle-mounted devices is proposed (refer to patent literature 1).
Today, in Intelligent Transport system (ITS), two kinds of wireless channels: a control channel (CCH) used for sharing information with a high priority such as an ID, location information, and the like of both devices and a service channel (SCH) used for transmitting and receiving a wide range of information other than the ID, the location information, and the like are used and one of two kinds of wireless channels is selected and used. The use of two kinds of the wireless channels means the use of two sets of transceivers. Accordingly, two sets of transceivers are installed in each of the roadside device placed along the road or the like and the vehicle-mounted device mounted on the vehicle. One of the transceivers is used for the CCH and the other is used for the SCH. In this case, the transceiver used for the CCH or the SCH is not specified.
In the following explanation, a vehicle to be targeted is referred to as a target vehicle, a vehicle with which this target vehicle communicates is referred to as a counter vehicle, and a vehicle which relays communication is referred to as a hopping vehicle. Further, the roadside device is referred to as an access point.
In such ITS, the provision of service information using the SCH is studied. As such service information, information used for driving a vehicle such as information about traffic jam, information about weather conditions, information about accident, and the like (hereinafter, referred to as driving-related information) and information about the Internet and entertainment (for example, a TV program or the like) (hereinafter, referred to as content-related information) may be supplied and these information are transmitted by a packet. Each of these information is transmitted by using a different broadcasting frequency. Therefore, a user has to tune the frequency of the SCH to the broadcasting frequency in order to acquire the information desired by the user. Hereinafter, the channels through which a plurality of service information are provided are described as SCH_1, SCH_2, and the like.
However, as mentioned above, because only two sets of transceivers are installed and one of two sets is reserved for the CCH, the remaining one set has to be used for the SCH. Accordingly, in a case in which the target vehicle and the counter vehicle perform vehicle-to-vehicle communication by using the SCH_1, when the target vehicle changes a channel from the SCH_1 to the SCH_2 to receive the service information provided on the SCH_2, the vehicle-to-vehicle communication between the target vehicle and the counter vehicle cannot be performed.
Accordingly, in Japanese Patent Application Laid-Open No. 2011-193096, a configuration in which when the target vehicle changes a channel from the SCH_1 to the SCH_2, the target vehicle notifies the counter vehicle of information indicating that the channel is changed is disclosed.
By using this configuration, while performing vehicle-to-vehicle communication between the target vehicle and the counter vehicle, the target vehicle can receive the service information provided on the SCH_2.